NHS bosses have criticised the British Medical Association for its "increasingly hardline" approach in rejecting emergency requests for striking doctors in England to return to work.
A system known as 'derogation' is in place whereby the NHS can ask for resident doctors, who are taking part in a five-day walkout, to cross the picket line where patient safety is at risk.
As of Sunday evening, 18 requests by hospitals for derogations had been rejected in this strike - the twelfth in the long-running pay dispute - with nine accepted.
The BMA said while it was ready to respond to emergencies, poor planning and the push to continue non-urgent care in this strike had stretched staffing too much.